Bikes with two or three chainrings are the norm, and then there are singlespeeds, but except for kids' bikes, you see almost no stock bikes with a single chainring in front but a cassette and derailleur in the back. It seems like for a lot of people this would be a nice balance with respect to simplicity, weight, maintenance, so why are there so few?

I think you're just not looking in the right places. Among the bike-crazed, "fixies" are all the rage. And many bikes these days come with an internal-gear rear hub and only one front sprocket. It doesn't happen more mostly because, for a standard derailleur-style bike it doesn't make sense -- you get a lot of added function with the extra front sprockets.
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Daniel R HicksAug 31 '12 at 19:30

My bike is like that: 1 ring in front, 8 in the back. I love this for the simplicity, plus I don't get dirty from the front ring because it's not "naked". I only use 6 of the gears and even that is perfectly fine for my 12km commute - and I'm one of the fastest non-racers on that route.
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Torben Gundtofte-BruunAug 31 '12 at 19:50

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1x9 setups are common on cyclocross bikes, also. You're generally faster sprinting uphill with a bike than spinning away in the mud.
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WTHarperSep 1 '12 at 2:03

4 Answers
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1x9 setups are more common on a mountain bike or commuter bike setup with rapidfire shifters than a drop bar setup. However, due to a lack of front derailleur they can have chain jump issues due to the effect of the rear derailleur on the chainline. This has to be compensated for, often with a chain guard on the outer side and jump stop on the inner side of the front chainring. It's also a bit weirder to have 2 STI brifters and only have one of them for shifting on a road / cross bike. Especially when brifters cost a lot of money.

Most groupsets come with all the drivetrain and shifting gear for a 2x9 or 2x10 setup. So, they install it on the bikes as a set, It would probably be cost prohibitive for manufacturers to break up the groupset and they would probably have to pay for extra parts (like a chain guard) to make it work.

There are however several bikes on the market with a single front chainring and an internally geared rear hub. These bikes don't suffer the downfalls of having to deal with chain jumps as much as bikes with derailleurs since the chainline stays consistent (and hopefully releativly straight).

I suspect marketing. More is always better. If you were naive to bikes and you saw two derailleur bikes for $499: One with 9 gears (1 x 9) or one with 21 gears (3 x 9). Which do you think they will choose? In western society, more is always and without question better. The single chainring bike might have better components (due to being simpler at the same price point), but people will fixate on the number of gears.

That said, I have noticed quite a few 8 speed hub internal hub bikes on the market now. Single chain ring with essentially the same number of gears as a single chainring derailleur bike, but they can do a marketing end run around the issue of number of gears with a different sell, namely the reduced maintenance and simplicity associated with an internal hub.

Aside - I will avoid any discussion of whether or not internal hubs are simpler and lower maintenance, just that this is the common "knowledge" about internal hub bikes.

I think that the argument "in western society, more is always and without question better" is an oversimplification. For many people, a bike with 21 gears truly is better than a bike with 9 gears. Sure, some people make mindless purchasing decisions, but there are usually more compelling rational arguments for explaining product availability.
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amcnabbAug 31 '12 at 19:31

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@amcnabb - The question explicitly states that author believed a single chainring and derailleur is enough for most people. I many flat areas I would agree. I know many of reps within the cycling industry, often times marketing decisions/sell jobs are really as shallow as more is better.
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Rider_XAug 31 '12 at 22:59

"one with 21 gears (3 x 9)." This one must be very bad, there's a lot of combinations that aren't possible ;)
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jv42Sep 5 '12 at 13:33

@jv42 - Ha ha! If only non-bike geeks actually knew this... Instead we get "What do I push to change a gear again?"
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Rider_XSep 5 '12 at 15:01

There are plenty of bikes available with a single chainring. However, only certain types of bicycles come with single chainrings. People generally buy according to their needs, and products are generally offered in accordance with demand.

The two main categories of bikes that often come with a single chainring are comfort/city bikes and track/single-speed bikes. Comfort bikes such as beach cruisers often have coaster brakes, chain guards, and other features to minimize the amount of maintenance required. Track bikes were originally designed for racing at velodromes, but they have developed a broader appeal due to trendiness and low maintenance requirements.

On the other hand, road bikes and mountain bikes generally have a wide range of gears to allow a wide range of speeds. If they came with only a single chainring, they would not meet the needs of most cyclists, so they wouldn't be bought.

In short, there are plenty of bicycles with single chainrings, and they tend to be bikes that are designed to be low maintenance. Bikes designed for performance usually come with a wide range of gears.

A wide range of gears on the rear is 11 T to 34 T which lets you vary your mechanical advantage by about 310% on a bike with only a rear derailleur.

The front derailleur is (compared to the rear derailleur) a simple piece of equipment that greatly increases your ability to go fast on the flats while climbing comfortably. Adding a front crankset that goes from 24 T to 52 T lets you vary your mechanical advantage by ((52/11) / (24/34)) or 670% with both derailleurs between large-front/small-rear and small-front/large-rear.

(The actual mechanical advantage has to take into account the ratio between the pedal circle circumference and the drive-wheel circumference, but I'm dividing mechanical advantages here so that constant divides out.)

Bikes with only a front derailleur aren't made today because they don't pick up the slack in the chain, though I've heard that early model chained bikes were often ridden with slack chains.